There are two general types of AGV guidance systems, rail systems and non-rail systems. In the first type of system, a pathway is formed out of rails. These rails may support an AGV or merely guide the AGV's wheels as they roll along the ground. In a non-rail system, the AGV may include detectors for detecting and following a wire in the ground that marks out a pathway, or a controller for following a set of commands to navigate between various types of reference markers. Each of these systems has certain advantages and drawbacks, and the choice of which system to use is normally based on many factors such as the needs of the particular user and the environment in which the system will operate.
Rail guidance allows for precise control over the position of an AGV. Where only a limited number of pathways are needed, and where these pathways do not need to be changed frequently, rail guidance offers a relatively simple method of keeping an AGV on a selected path. One of the biggest drawbacks to rail guidance, however, is that switches are needed to direct AGV's from one pathway to another. These switches are relatively costly and include moving parts that can wear out. In addition, each switch must be connected to a power source, a sensor for determining the position of the switch, and a controller for moving the switch from one position to another at appropriate times. The switches are often connected to a controller and to one another by a series of wires that run along the pathways, and these wires are expensive to install and maintain. Furthermore, the wires must be reconfigured each time the system is modified. Another disadvantage to such systems is that the rails themselves are generally raised off the ground and can interfere with the free movement of people and other vehicles.
Non-rail guidance systems offer increased design flexibility since pathways can be changed by reprogramming the AGV's or their controllers and without removing and re-laying rails. Moreover, because each vehicle receives or is programmed with instructions concerning the pathway to follow, switches are not needed to shift a vehicle from one path to another. However, because AGV's in such a system can stray from their pathways, extra care is required to make sure that each AGV is in its intended location and often this entails virtually constant communication with each AGV in the system. The quality of the communication link and the speed at which information about the AGV's and their positions can be processed also limits the maximum rate of travel of these systems. Moreover, collision avoidance becomes more complicated when AGV's travel along pathways that are not defined by rails. The need to constantly monitor and control a large number of AGV's, and to keep them on course and to avoid collisions, requires a significant amount of processing power which can make non-rail guidance systems more complex and expensive to operate than rail systems.
For high speed transport, that is for speeds in the range of 2200 feet per minute, rail guidance has traditionally been the only practical method for guiding an AGV. This is in part due to a perception that it is unsafe to operate vehicles at high speeds without physical path constraints and partly due to control problems. For example, the servo-control mechanisms used to steer AGV's often cannot respond quickly enough to the changing location of a guide wire in order to control a fast-moving vehicle. In addition, the signal to noise ratio of the position sensors may be too low to allow them to accurately sense the presence of a wire in the ground or to communicate reliably with a central controller when moving rapidly. Therefore, in applications where high speeds are needed, it has heretofore been necessary to use rail based control with all of its attendant drawbacks.